Chap. XV] DIGESTIVE PROCESSES 301 



and during this time the feces continue to lose water, become 

 harder, and more diflficult to expeL 



ABSORPTION 



This is the process by means of which the digested food is 

 taken from the intestines and carried into the blood. We have 

 now to consider this process, for, properly speaking, though the 

 food may be digested and ready for nutritive purposes, it is, 

 until it passes through the walls of the alimentary canal, still 

 practically outside the body. 



Absorption. — Absorption is a very complex process and may 

 be subdivided into a physical and physiological process. The 

 physical process consists of the passage of the digested food from 

 the intestines into the blood-vessels, and is governed by the laws 

 of diffusion and osmosis. The physiological process consists in 

 the building up of the end products of protein and fat digestion 

 into the substances found in the blood. This process of reconstruc- 

 tion is not understood, but is dependent on the living epithelial 

 cells that make up the intestinal walls. 



Paths of absorption. — There are two paths by means of which 

 the products of digestion find their way into the blood : — 



(1) By the capillaries in the walls of the stomach and intestines. 



(2) By the lymphatics in the walls of the small intestine (the 

 lacteals) . 



It is now thought that absorption through the stomach is 

 limited to small quantities of such substances as water, glucose, 

 and salts. This means that the greater part of absorption is a 

 function of the small and large intestines. The products that 

 result from the digestion of proteins and carbohydrates pass into 

 the capillaries. The products resulting from the digestion of 

 fats pass into the villi of the small intestine. 



